3) B

Total number of screws = 68 + 10 + 2 + 22 + 20 + 18 + 10 + 54 + 46 = 250.

Number of 4 mm screws which have length of either 35 mm or 45 mm = 20 + 18 = 38

Probability = (38 / 250) x 100 = 15.2 ~ 15%

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